1. Field of the Invention
The present invention relates to a discharge head and an image forming apparatus, and more particularly to a structure and manufacturing technology for a piezoelectric element which is used in a discharge head contained in an image forming apparatus such as an inkjet recording apparatus.
2. Description of the Related Art
Conventionally, as one example of an image forming apparatus, an inkjet recording apparatus is known in which comprises an inkjet head (a discharge head, or a discharge head) having an arrangement of a plurality of nozzles (discharge elements), which records images on an image forming medium by discharging ink from the nozzles while causing the inkjet head and an image forming medium (a discharge receiving medium) to move relatively to each other.
There are various ink discharge methods for inkjet heads in such the inkjet recording apparatuses. For example, one known method is a piezoelectric method, in which the volume of a pressure chamber is changed by causing a diaphragm forming a portion of the pressure chamber to deform due to deformation of a piezoelectric element, ink being introduced into the pressure chamber from an ink supply passage when the volume is increased, and the ink inside the pressure chamber being discharged as a droplet from the nozzle when the volume of the pressure chamber is reduced. Another known method is a thermal inkjet method, in which ink inside an ink chamber (a pressure chamber) is heated to generate a bubble in the ink, and ink is then discharged by means of the expansive energy created as the bubble grows.
In an inkjet head comprising piezoelectric elements as pressure elements which apply a discharge pressure to the ink inside the pressure chambers, the materials, structure, shape, and the like, of the pressure chambers, piezoelectric elements, and other parts, are designed variously, for instance, layers having components of different compositions or mixed layers comprising a combination of a plurality of components are used in piezoelectric elements having a multiple-layer structure, in order to be able to ensure satisfactory discharge of ink, even when using ink of high viscosity which requires a large discharge pressure, by applying the discharge pressure to the ink inside the pressure chambers with good efficiency.
In an inkjet recording apparatus, if gas bubbles occur inside the inkjet head due to infiltration of air, or change in the temperature of the ink, then the change in volume of the pressure chambers is absorbed by the gas bubbles, and hence it is not possible to impart a sufficient discharge pressure to the ink and discharge abnormalities in which ink discharge is incomplete may arise. Furthermore, a discharge failure in which no ink droplet is discharged from the nozzle may arise in the case of blocking a nozzle by drying of the ink in the vicinity of the nozzles or due to foreign material, such as dirt, or the like, or in the case of failure to replenish ink into the pressure chamber.
The occurrence of discharge abnormalities or discharge failures of these kinds reduces the performance of the inkjet head, and causes abnormalities in the shape (size or form) of the dots formed on the image forming medium, or omission of such dots, thus leading to a decline in the quality of the image formed on the image forming medium. Therefore, image quality can be maintained by rapidly determining the occurrence of a discharge abnormality or discharge failure and performing maintenance, such as purging, at the nozzle producing the discharge abnormality or discharge failure, thereby ensuring that ink droplets are discharged in a desirable state from each of the nozzles.
Conventionally, methods for determining various kinds of discharge abnormalities and discharge failures have been proposed, such as a method for determining the occurrence of a discharge abnormality or discharge failure such as that described above, from the state of the pressure (vibration) inside the pressure chamber, or a method for determining the occurrence of a discharge abnormality or discharge failure from the image (dots) formed on the image forming medium.
The ink discharge apparatus disclosed in Japanese Patent Application Publication No. 55-118878 is an ink discharge apparatus in which a portion of the walls forming an ink chamber having an ink supply port by which ink is supplied from an ink tank and an ink droplet discharge port by which ink is propelled as a particle is constituted by a vibrating element which causes the volume of the ink chamber to change by being displaced in response to an electrical signal, which comprises a determination device for determining the state of displacement of the vibrating element so as to detect an abnormality in the displacement state of the vibrating element with respect to the electrical signal. Since a high-frequency wave is superimposed on the electrical signal which determines the displacement state, when there is a discharge failure, then a discharge abnormality is judged to have occurred when a high-frequency wave component is included in this electrical signal.
Furthermore, in the piezoelectric element, inkjet recording head and manufacturing method thereof disclosed in Japanese Patent Application Publication No. 11-238920, piezoelectric elements are constituted by laminating together a plurality of piezoelectric thin films between an upper electrode film and a lower electrode film. In this piezoelectric element, microcrystalline particles are dispersed within the respective piezoelectric thin films. In addition, the surface density of the microcrystalline particles in the respective piezoelectric thin films tends to decrease, as the distance increases from the piezoelectric thin film which is in contact with the lower electrode film. If microcrystalline particles which are different to the original crystals are grown in the respective layers of a piezoelectric body comprising a plurality of layers, and if the piezoelectric element is composed in such a manner that the density of microcrystalline particles declines toward the upper side layer, then the microcrystalline particles alleviate the internal stress. Therefore, since the piezoelectric body can be formed as a thick film, it is possible to improve the reliability thereof.
Furthermore, in the inkjet printer head disclosed in Japanese Patent Application Publication No. 2001-129993, a method of forming a film by blowing fine particles onto a substrate from a fine nozzle at a high speed of several hundred m/sec. (namely, aerosol deposition, or AD), is adapted as the method of forming the piezoelectric elements of the inkjet printer, for manufacturing inkjet drive elements to an optimal film thickness accurately in a short period of time.
Moreover, the piezoelectric thin film element disclosed in Japanese Patent Application Publication No. 2000-22233, comprises a piezoelectric film sandwiched between an upper electrode and a lower electrode. This piezoelectric film has a structure composed of a plurality of components in which regions that are adjacent in the direction perpendicular to the thickness of the film (the width direction) have mutually different compositions. By adjusting the ratio occupied by each of the respective components constituting the piezoelectric films in the piezoelectric film, it is possible to adjust the dielectric constant and the piezoelectric g constant of the piezoelectric film, and hence the piezoelectric d31 constant can be improved in comparison with a piezoelectric body having a simple composition.
As described above, the inkjet heads relating to the conventional technology are disclosed as a technology for improving discharge efficiency by generating a force from a piezoelectric body with good efficiency, or as a technology for improving reliability by reducing the internal stress of the piezoelectric body and the peripheral members. However, the inkjet heads relating to the conventional technology are not disclosed as a technology for forming drive elements (pressure elements) which serve to eject ink and determination elements which serve to determine the ink discharge state as integral bodies, and as a technology for improving both discharge efficiency and determination efficiency, in an inkjet head that is integrated to a high density.
In the ink discharge device disclosed in Japanese Patent Application Publication No. 55-118878, a portion of a vibrating element (drive element) constitutes a determination element which determines the displacement of the vibrating element (i.e., the vibrating element has a structure in which drive elements and determination elements are arranged on a diaphragm plate). Since those elements have independent electrical connections respectively, it is not suitable for a high-density structure.
In the piezoelectric element, inkjet recording head and manufacturing method thereof disclosed in Japanese Patent Application Publication No. 11-238920, the piezoelectric element has a structure which comprises different compositions, and the like, in the thickness direction of the piezoelectric body, so that the drive properties (the properties relating to ink discharge) can be improved. However, when the piezoelectric body serves both to drive ink discharge and to determine the state of discharge, there is no disclosure regarding a suitable method.
In the inkjet printer head disclosed in Japanese Patent Application Publication No. 2001-129993, the AD method is disclosed as a technology for forming drive elements (piezoelectric bodies), but there is no particular disclosure regarding the determination of the discharge condition.
In the piezoelectric thin film element disclosed in Japanese Patent Application Publication No. 2000-22233, since the piezoelectric thin film element has a structure comprising different compositions, and the like, in a direction perpendicular to the thickness direction of the drive elements, it is possible to improve the drive properties (the properties relating to ink discharge) as a similar to the inkjet recording head and manufacturing method thereof described in Japanese Patent Application Publication No. 11-238920. However, when the piezoelectric body serves both to drive ink discharge and to determine the discharge condition, there is no disclosure regarding a suitable method.